Exploring Antimatter Physics and Future Applications: Professor Carsten Welsch

Interview conducted by Zoe PeterkinSep 3 2019

What are your focus areas for research at the University of Liverpool and the Cockcroft Institute?

The University's research focus areas are very broad. In the Physics Department, we work across all aspects of modern physics, including nuclear physics, condensed matter physics and particle physics. Our research is tackling all of the big questions that physics is trying to answer.

The university also plays a key role in the Cockcroft Institute that was founded just over 10 years ago and is based in the North West of England. It's a collaboration between four partner universities: Liverpool, Manchester, Lancaster, and Strathclyde in Scotland, as well as the Accelerator Science and Technology Centre (ASTeC) within the Science and Technology Facilities Council (STFC).

The Cockcroft Institute’s mission is to develop particle accelerators for the benefit of science and society. It focuses entirely on R&D into particle accelerators and related technologies.

My own research interests lie in the design and optimization of accelerators and light sources, with a focus on the development of beam diagnostics that help characterize beams of charged particles and beam dynamics where we design and optimize existing and future accelerators.

An adaptive optics-based beam monitor for characterizing beams of (anti)matter is being developed within the AVA project.
Image credit: University of Liverpool/Cockcroft Institute.

What are some of the challenges surrounding the field of antimatter research at the moment?

The first challenge is that there is no free antimatter available anywhere in the universe. This is one of the big mysteries of physics as it clashes with some very fundamental assumptions and laws that we believe govern our universe.

At the beginning of time, an equal amount of matter and antimatter should have been created, and at some point, these particles should have met again and annihilation should have occurred. The only thing that should have been left is light. Fortunately, however, that didn't happen and we are here.

However, this means that at the moment, we are only aware of matter in our universe – there is no sign of any antimatter available anywhere. That brings up a very fundamental question - why is that the case? Why is the universe not symmetric? Why has there been this shift towards the matter universe?

There are a number of different ways to produce antimatter. One way is to use radioactive isotopes that occur in nature. If you have an unstable isotope that decays, it might also emit antiparticles such as positrons. This is an effect which is actually used in clinical practice - for example, when doctors inject radioactive isotopes into patients. As the isotope decays, it emits a particle and an antiparticle, which can be used for positron emission tomography (PET). This can be used to image, for example, cancer cells in the body of a patient.

Producing antimatter for fundamental science studies is a little more complicated. We use one of the best known physics equations: Einstein's E = mc². This states that as long as you bring enough energy to a single point in space, you can create (anti)matter. The equation doesn't say anything about what type of matter is created, but when you bring a lot of energy to one point, you will create antimatter particles as well as matter.

How to particle accelerators help researchers produce and study antimatter?

The recipe for producing antimatter in the lab starts with taking a beam of charged particles and making it very high energy. You need an accelerator to bring a beam of protons to that level of energy, which you can then use for further studies in lab experiments.

The protons are then fired onto a metallic target, which is used because metal can withstand a high heat load and doesn't melt. As the proton beam is fired onto the target, you bring a lot of energy into a single point in space. That collision will create all sorts of particles, including antiparticles.

Next, amongst all the different particles that were created in this process, you need to extract the antiparticles. This is done using a combination of electric and magnetic fields, which filter out the antiparticles. The antiparticles are actually only a small percentage of the total particles that were produced - for every one million protons that impact on that metallic target, only one antiproton is typically produced. It is a very low yield, but nevertheless it allows you to produce antimatter, up to 10 Million antiprotons per shot on the target.

A problem that arises after you have created antimatter using a high energy beam, is that the antiparticles are also emitted with very high velocity so they cannot be used for precision studies. Ideally, to study a particle in great detail, it needs to be at rest. Once can then use lasers to study its energy levels or measure how it behaves in the gravitational field of the earth.

To get an antiparticle to its resting state, you need to use a particle accelerator in reverse; decelerating the antiproton from a very high energy until ultimately you bring it to rest. 'Rest' in our case means that they are injected into an ion trap, which is essentially a vacuum chamber with electric and magnetic fields. These fields make sure the antiparticles inside the cage don’t touch any of the surrounding walls, because as soon as an antiparticle comes into contact with matter, it annihilates.

Once we have the antiprotons in the ion trap, we can overlap them with a cloud of the antiparticle of the electron, the positron, creating, antihydrogen. Antihydrogen is the mirror particle of hydrogen, and is one of the basic building bricks of our universe It allows us to study in detail why, or if, there is any difference in the way that matter and antimatter work.

How does studying antimatter physics create benefits for society?

In terms of the scientific knowledge, it is very disturbing that we don't know why we are here. Why is there this matter universe? And why is our very fundamental understanding of the universe wrong at the moment? It is something we need to address.

Previously, nobody had ever measured whether an antiparticle in the gravitational field of the earth falls down or goes up. We still do not know all of the differences between matter and antimatter. We have no idea experimentally about some of the most fundamental aspect of our universe.

ELENA storage ring where antiprotons are decelerated to lowest energies"
Image credit: CERN.

In terms of applications for antimatter, there are a number of different areas where they are used routinely. I mentioned the PET imaging used in hospitals earlier, which has been in clinical practice for decades, and is used on a daily basis.

There have also been studies at CERN to use antiprotons for cancer treatment processes. One of the most advanced cancer treatment technologies, which has just come over to the UK, is proton beam therapy. If you were to replace the protons with antiprotons, there might be some distinct advantages. For example, the energy ratio between the tumor region and the entrance channel could be even more advantageous in the case of antiprotons. Professionals could also image the beam while carrying out the treatment, benefiting from the signals generated when the antiproton beam annihilates in the body of the patient. However, it is an application that is still being investigated in the lab and far from clinical application.

There have also been proposals to use antiproton beams for the propulsion of spacecraft. NASA is now considering sending people to Mars and who knows, at some point there might be explorers who want to go even further. Then the question is, what kind of engines will drive the spacecraft? The beauty about antiparticles is that the energy density is extremely high, because the annihilation efficiency which converts antiparticles into light is 100%. Therefore, for everything that you use in your engine, you would get shear power out of the reaction. That's a thousand times higher than any chemical fuel or nuclear reaction.

How does antimatter research need to develop to reach these future applications?

The big challenge of antiproton physics research is that there is only one place in the world where you can do that type of study: CERN in Geneva. That means that there’s only one place on the whole planet where you can ask these questions and hence a high competition to get access to the beams there. Furthermore, the experiments are very difficult to do because the process of producing antimatter is so inefficient.

I mentioned that only one antiparticle comes out of the reaction for every one million protons, but the total number of protons that you fire onto this block of metal is also limited. In reality, what you get is a maximum of ten million antiparticles every two minutes. This is because the proton beam has to be created, accelerated, and guided towards the target. Then you can create the antiparticles, but these have to be slowed down, and in that time you can't produce more particles. And all of this is only possible when the main accelerator for the protons is running – which is not always the case.

Antimatter is one of the most expensive materials in the world. That makes experiments very difficult because there is a lot of competition for these beams and very few experiments that get the permission to carry out the measurements. It also means that progress is very slow; we're talking decades rather than years.

Other than CERN, there's a facility being built in Germany called the Facility for Antiproton and Ion Research (FAIR), which will serve the global research community from 2025. It has antiparticle research built into its science program. However, it currently would not cover low-energy studies, although it could be an option in the future. Other than that, I'm not aware of any plans for another low energy antimatter facility anywhere in the world.

You coordinate the EU-funded Accelerators Validating Antimatter Physics (AVA) - what are the aims of this network and how does it work?

Taking into account all of the things I mentioned before - experiments that are very difficult to do, a unique and hard-to-access place where you can do these studies - it makes for a very demanding area of research. At the same time, it offers a unique opportunity for training the next generation of scientists, because scientists and engineers are usually attracted by challenges. If you give them a difficult challenge, they will grow with that challenge and become even better.

A few years ago I had the idea of setting up an international network of experts to address all of the major challenges around antimatter physics. We work together collaboratively to train 15 researchers at institutions all across Europe. They work together to develop next generation techniques and technologies for antimatter research and help further improve the research infrastructure at CERN. My idea was that this will provide them with a rather unique challenge, as well as a good basis for their training.

AVA aims to train these early career researchers and provide them with a very broad range of skills. This should be a good basis for their future careers. The AVA project also provides a lot of scientific events like workshops and schools about antimatter physics for the wider scientific community. We have also organized large scale outreach activities like a big symposia, where we engage school children and the general public with our research to create more awareness about the challenges and the importance of antimatter. We have also produced a film about the project which quickly became the most-watched science short film on the European Commission’s official YouTube playlist.

AVA stands for Accelerators Validating Antimatter Physics, but the acronym is also because of a little girl called Ava from Warrington in the UK who sadly died from cancer a few years ago. I previously mentioned the cancer applications that may become possible with antiproton beams. In discussion with Ava’s family, we connected the project to the name of the little girl, to keep her memory alive.

About Professor Carsten Welsch

Professor Carsten Welsch studied physics and economics at the Universities of Frankfurt in Germany and UC Berkeley in the United States. He received his PhD in accelerator physics at the University of Frankfurt and after some years did Postdoc research at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. He was awarded a Fellowship at CERN in Switzerland. He founded the pan-European QUASAR Group in 2007.

Professor Carsten Welsch has been a member of the academic staff at the University of Liverpool and a member of the Cockcroft Institute of Accelerator Science and Technology since 2008. In 2011 he was promoted to Full Professor of Physics and has been Head of the Physics Department since September 2016.